The present invention relates to a current sampling output driver that is capable of sensing and adjusting to accommodate a wide range of current loads. More specifically, the invention is a half-bridge output driver designed to interface an inductive or capacitive load, typically a motor, to a low voltage logic device in order to detect and control of the speed, power, and torque of a motor.
DC motor performance varies in relation to motor current (Imotor), motor torque (M), motor speed (n), and output power (Pout), assuming negligible internal friction. The motor current, Imotor, is a linear function of the motor torque, M. Imotor=ƒ(M), or xcex94Imotor=xcex94M. Increasing current to the motor, Imotor, increases the motor""s torque. Motor speed, n, is an inverse linear function of the motor torque, M. n=ƒ(M), or xcex94n=xe2x88x92xcex94M. Motor speed, n, increases as motor torque, M, decreases. Finally, the output power, Pout, is determined by the applied motor current, Imotor, multiplied by the motor generator Electro Motive Force, VEMF. Pout=Imotorxc2x7VEMF, or Pout=I2motorxc2x7R. It may therefore be seen that the speed, power, and torque of the motor may all be controlled by controlling either the current or voltage delivered to the motor.
DC motors function as both resistors to supply voltage and current and also as generators, creating a voltage described as xe2x80x9cback electro magnetic forcexe2x80x9d or BackEMF. By placing a relatively small resistor, as sensing resistor, in series with the load, and measuring the voltage across this resistor, it is possible to determine the current through the resistor and thereby to the load. The current through the resistor is changed depending on the BackEMP from the motor. The measured signal from across the sensing resistor is directed to a differential amplifier. By comparing the sensed current or voltage from the resistor and the motor, the differential amplifier can create a signal reflecting the operational condition of the motor and convey this signal to circuit control logic.
Typically, the status of a DC motor operating within a set current is sensed by a xe2x80x9ctransduction amplifier,xe2x80x9d which is so called because it maps voltages to currents or current to voltages. A typical transduction amplifier for DC motor control is a differential operational amplifier which typically has a voltage input mapping to a current magnitude. The voltage, or current, in a DC motor circuit is read by a differential amplifier as determined from reading voltage from the sensing resistor in series with the load. A voltage proportional to motor current, which is the inductor current, is generated by the sensing resistor and the differential amplifier. The sensing resistor voltage is then compared against a reference voltage. Again, a problem with this type of control is that a differential amplifier operates only within a specific voltage range. It is therefore necessary to keep the system voltages within that range.
A conventional solution to controlling the voltage to the differential amplifier is to increase or decrease the resistance of the circuit in relation to the decrease or increase of the applied current. From Ohm""s Law, v=ixc2x7R, it is well known to change resistor values to keep a constant voltage with changing current. The problem with using a conventional circuit is that current required to control the motor, as applied to the sensing resistor, may exceed the operational voltage range of the differential amplifier. Therefore, the circuit is limited to a small current range for any selected resistor value. A conventional solution to this problem has been to physically switch resistors in and out of the circuit to accommodate changing current. The physical switching of the resistors creates it own problems of error introduced by the added resistance of the switching mechanisms or devices themselves.
For a set motor current, the applied voltage is controlled by Pulse Wide Modulation, PWM, of the source drivers within a pre-set hysteresis format. Load current is monitored through an inline series resistor that is constantly monitored by the differential amplifier. Then the current reaches a set point, as determined by the feedback analysis and motor control logic, stages may be selected in or out of the circuit to permit a change of current, yet maintaining the appropriate sensing voltage range for the differential amplifier.
It is therefore a principal object of this invention to provide a motor driver with a predictable sensing resistance value over a broad range of dynamically changing current magnitudes.
These and other objects will be apparent to those skilled in the art.
A current sampling half-bridge output driver capable of sensing and adjusting to a range of currents; having a plurality of selectable stages of current sensing circuits with a plurality of sensing resistors operatively connected between the stages; wherein the signal across the sensing resistors varies with the feedback signal from the motor and the variation is detected by a differential amplifier which is in communication with a logic device capable of connecting or disconnecting current sensing stages to adjust current to the motor. The series resistors in combination with the selectable stages make it possible to dynamically and predictably adjust the driver to a wide range of currents.